1. Field of the Invention
The present invention generally relates to a method of forming an image, and more particularly, to a method of forming a reversal image by utilizing an electrophotographic image forming process.
2. Description of the Prior Art
Conventionally, both positive and negative microfilms are each used as an original document in a reader printer or the like. Accordingly, such an apparatus is provided with both the function for regular development and that for reversal development so that a positive image or a negative one may be obtained from either the positive microfilm or the negative one.
However, a copying apparatus is only capable of copying in the regular development in which a positive image is formed from a positive original document. If a negative image can be obtained from a positive original document in the copying apparatus, a variety of images being attractive in design can greatly enhance the utility value of the copying apparatus.
However, if both the functions of regular and reversal developments are given to the copying apparatus, the apparatus is inevitably formed into a large size undesirably at an increased cost.
More specifically, if a photoreceptor drum is electrostatically charged in either the positive or negative polarity, it is necessary to selectively use two kinds of differently polarized toners for the regular or reversal development, with these toners contained in respective developing devices. Furthermore, two high-voltage power sources are necessarily required to switch the polarity of electric voltage applied to a charge wire of a transfer charger.
If the copying apparatus is provided with a photoreceptor drum which can be electrostatically charged selectively in either of both polarities, it is necessary to provide two chargers and two high-voltage power sources for switching the polarity of electric voltage applied to the charge wires of respective chargers.